miniature thick resistors for...

Active and Passive Elec. Comp., 1991, Vol. 14, pp. 119-127Reprints available directly from the publisherPhotocopying permitted by license only

1991 Gordon and Breach Science Publishers S.A.Printed in the United Kingdom

MINIATURE THICK FILM RESISTORS FORMICROWAVE INTEGRATED CIRCUITS

ZBIGNIEW SZCZEPAlqSKIWarsaw University of Technology, Institute of Microelectronics and Optoelectronics

(Received May 29, 1990; in final form August 23, 1990)

In this paper the results of miniature thick film resistors designed as add-on lumped resistors arepresented. A few resistor configurations have been elaborated and tested, and a specific trimmingresistance, which avoids parasitic trimming inductance was used. Some of the designed resistors wereused in isolators and directional couplers for the 2-11 GHz band. The electrical parameters have beenpresented and discussed. The assembly methods for these resistors to the microstrip lines have alsobeen given.

INTRODUCTION

Microwave integrated circuits (MIC) technology has progressed rapidly in the lastyears. At the same time, many new kinds of active and passive components forthe MIC’s have been developed. Presently most widely used construction of themicrowave integrated circuits are hybrid microwave integrated circuits, based onmicrostrip lines, which play the role of a wave guide. In these circuits, except forthe distributed elements, the lumped-elements such as resistors, capacitors anddiodes often exist.

The resistance MIC’s elements can be used as isolators load, in attenuators, asmatching elements, and as shunt to ground 500 terminations. The resistors forMIC’s should guarantee a low temperature coefficient of resistance (TCR), goodresistance stability, small dimensions, easy attachment to the microstrip line, andample power handling.Low TCR and good stability involve small time and temperature resistance

changes. The resistor sizes for microwave application should be very small. Whenthey are much smaller than the applicable wavelength, the resistors behave likelumped-element resistors, at which the parasitic effects are also minimized. As arule the largest dimension of the resistor should be less than 0.1 2 at the highestfrequency.

But one should note that too small dimensions may cause some processing andmanufacturing problems. Larger dimensions allow greater power dissipation butthe maximum resistor dimension is limited by the increasing parasitic. Anotherproblem concerning a strip line dimension is the continuity of transmission linewhere the resistor is connected. It would be desirable that the resistor width ap-proximated the strip line width.

119

120 Z. SZCZEPAlqSKI

Since the limiting frequency of a skin effect for a thick resistive film is very largeand an electromagnetic field penetrates the whole resistive film thickness, theresistance value can be described as"

LR=Ro ,,,=ZoThere: Ro is the sheet resistivity, and L and W are the film length and width,respectively. The resistance value can thus be defined by counting the number ofsquares between the conductors pads. When a resistor is attached to a strip lineas the matching element, its resistance should be matched to the input strip lineimpedance Zo. Thus these resistors must be made with a good resistance tolerance.

EXPERIMENTAL

The choice of the configuration and resistor technology for the microwave resistorswas determined by such factors as the need for miniaturization of such resistors,low production cost, good resistance stability, high power carrying capacity ofresistive film, and good resistance for environmental conditions.

Taking into account different applications of these resistors and different op-erating frequencies, a few resistor configurations have been designed:

Plane configurationSpatial configurationMixed configuration.

These configurations differ from the distribution of conductor pads as well as fromdifferent dimensions.

For the realization of these resistors, thick film technology has been chosen,since thick resistive films based on ruthenium have been characterized by highpower handling capability (up to 15 W/cm:). It allows the dissipation of considerablepower on a small surface of resistive film.

All the manufacturing processes of thick film resistors have been based on Polish-made materials, as well as substrates and conductive and resistive compositions.The realized resistors have been tested in order to define electrical and RF per-formances.

All resistors were examined for long term stability, TCR, VSWR, and powerhandling capability. Before measurements of the resistors, they were aged at 150Cfor 36 hours to stabilize their properties; failures were eliminated at this stage.

The main advantage of designed resistors is the specific applied trimmingmethod4. Resistors are often trimmed by laser trimming or air abrasive trimmingtechniques. The laser trimming increases inductance at microwave frequencies sincethe microwave signal path becomes longer. (Fig. 1). On the other hand, lasertrimming reduces power handling capability. Therefore laser trimming cannot beused for resistors to be operated at high power dissipation and high frequencies2.

MINIATURE THICK FILM RESISTORS 121

FIGURE The effect of laser cut for RF performance resistor: a--resistor with laser cut; bmresistorwithout cut.

trace

FIGURE 2 The scheme of plane resistor configuration.

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-O2’BOO 00’" i., 200

300 400 500 ’6’00 700"

FIGURE 3 Long-term resistance stability for plane resistors: a--at 398K without load; bmat 343Kwith electric load.

--o "o

o"c TEC]

FIGURE 4 Resistance changes as a function of temperature.

122 Z. SZCZEPAlqSKI

Trimming by means of changes in the resistive film thickness cannot be used either,since it decreases the resistance stability.

To achieve a good tolerance of resistance, the designed and fabricated resistorswere trimmed by thermal trimming through one, two or several stages. This methodpermits a resistance tolerance to 0.5 per cent. It must be noted that with thermaltrimming, the resistors after firing should have tolerance of not more than 10-15per cent. In this case thick film processes will require more careful preparation ofboth conductor and resistor artwork.

In all the designed and fabricated resistors, the resistive film was coated with aglass composition, which affords resistors a protection from environmental con-ditions.

In Fig. 1 the effect of laser cut for VSWR can be seen.

4.2

4.’15-

VSWR

FIGURE 5 VSWR vs. frequency for plane resistors.

FIGURE 6 Spatial resistor configuration.

FIGURE 7 VSWR vs. frequency for spatial resistors.


FIGURE 8 Long-term resistance stability and the changes of TCR with temperature for spatialresistors.

EXPERIMENTAL RESULTS

The first configuration (Fig. 2) is characterized by the resistive and conductivelayers that are located on the same surface. Such a shape avoids parasitic capaci-tance connected with resistor. Attachment of the resistor to the microstrip line isachieved by soldering using the "flip chip" technique. Plane resistors are charac-terized by a good long term resistance stability as shown in Fig. 3.

The changes in termperature coefficient of resistance with temperature for theplane resistors are shown in Fig. 4.To define the resistors properties at microwave frequencies the resistors were

attached to .50 ohm transmission line with a 1/4 2 open circuit stub (Fig. 5). Thesecond configuration results in a high degree of miniaturization as resistive andconductive layers are spaced on three separate planes of a cubic body (Fig. 6). Thesmallest sizes obtained for such resistors were 0.7 0.7 0.5 mm. Such resistorsare recommended for higher band frequency since they are characterized by lowVSWR over the operating frequency (Fig. 7). Thermal measurements for spatialresistors are shown in Fig. 8. The measured resistive film temperature was below398K during the thermal resistance stability with an electric loading of 140 mW on

FIGURE 9 Mixed resistor configuration" amthe configuration with dual conductor pads; b--asym-metric resistor configuration.

124 Z. SZCZEPAlqSKI

every resistor. Because of the small sizes of these resistors, problems with theprecision position and placement arise with their attachment.

In the last configuration, i.e., mixed configuration, the conductive film is put ontwo or three planes of the body (Fig. 9). The resistors with double conductive padsare relatively easy to assemble. The configuration "8b" with one metal strip isespecially recommended when it is used as a shunt to ground (Fig. 10). The electricand microwave properties of the resistors with double conductor pads of 1.8 0.8

0.5 mm are given in Fig. 11 and Fig. 12. The general parameters for the fabricatedresistors have been compiled in Table 1.

FIGURE 10 The scheme of RF grounding achieved with metal strip.

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FIGURE 11 Long-term stability for mixed resistor configuration.

FIGURE 12 VSWR vs. frequency for mixed resistor configuration.

126 Z. SZCZEPAlqSKI

FIGURE 13 The scheme of chip resistors with metal strips.

MOUNTING RECOMMENDATIONS FOR CHIP RESISTORS

Designed and elaborated resistors are available as leadless chips or chips with metalstrips (Fig. 13). These metal strips make chip resistor attachment easy. They aremade of copper and are electroplated with gold. The metal strips are attached tothe thick film metalization with the use of microgap welding or eutectic solder withhigh melting temperature (i.e. Sn96Ag4, te 220C). When choosing an attach-ment technique, great care must be given to achieving the desired RF performance.

In all these cases 50 ohm microstrip lines are the most widely used for thesecomponents. Any device parasitic reactances can usually be compensated by ex-ternal circuitry. In general, it is essential to minimize the capacitance to groundfor attenuation of 1 to 8 dB values. This can be accomplished by a large lowdielectric ground plane spacing. In all cases, grounding is critical. For terminatingresistors the same rules apply to the ground connections. The parasitics associatedwith termination connections for in-line resistors should be minimized. This canbe realized by matching to the stripline width, both the chip pad sizes and theattachment method. Epoxy bonding and various soldering methods are mainly usedfor attachment of chip resistors.

Epoxy bonding is suitable for most applications. For the mounting of chip re-sistors, silver or gold conductive epoxies are used; silver epoxy is commonly useddue to its low cost and good bonding with palladium silver and platinum silverterminations.

The epoxy can be applied by screening or dispensing on the substrate surfaceto both leadless chip resistors and chips with metal strips. Chip resistor solderingcan be made with reflow soldering in which solder preforms, solder paste or pre-

circuit board/ k.,round plane

FIGURE 14 The scheme of the attachment of spatial chip resistor with metal strips.


tinning can be used. For chip resistors with a plane configuration, the flip-chiptechnique with pretinning of resistor pads is most often suitable.

The miniature chip resistors with a spatial configuration are attached to the stripline by the means of epoxy bonding or soldering with an iron. In this case, priorto making epoxy or solder electrical connections to the chip, it must be held inplace using non-conductive epoxy, ensuring that the chip does not move whensoldering.

The spatial chip configuration with metal strips is the most suitable when theresistor is located in the cavity of a substrate (Fig. 14).

The chip resistors with metal strips can be soldered to circuit board using solderwith paste or in wire form or melted by an iron, torch, or other localized source.

CONCLUSIONS

From the above experimental data, the following conclusions can be drawn"

Designed thick film chip resistors can be used in microwave integrated circuitsin the operating frequency up to X band

These resistors are characterized by good resistance stability and satisfactoryvalues of TCR

Chip resistors offer good RF performances up to 12 GHz with VSWR betterthan 1.22

They can be easily attached to microstrip lines by the means of conventionalassembly methods

Plane and spatial chip resistors have been used in isolators for L-S and X bandand the achieved parameters were satisfactory.

REFERENCES

1. Larry Boss; State of the Art in Hybrid Circuits Resistors. Hybrid Circuits Technology. vol. 6, No.10, October 1989.

2. Chen Y. Ho; VSWR, Power Dissipation: Keys to Film Resistors Microwaves. December 1981, pp.69-78.

3. G. Stecher, R. Bosch; Thick Film Resistors not only on Ceramics: How to Obtain Suitable LayoutParameters. Hybrid Circuits No. 17, September 1988.

4. Z. Szczepafiski, R. Kisiel, J. Kalenik; Thick Film Mini Chip Resistors for Microwave Applications.32. Internationales Wissenschaftliches Kolloquium. Ilmenau/DDR/, October 1988.

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